A power amplifier for power-amplifying an analog signal, e.g., an audio signal, must have a high power output efficiency in a large signal to noise ratio (S/N) condition, since the amplifier is used in general as an audio amplifier. An SEPP output stage consists of two amplifying sections connected to a common load circuit. One section of the SEPP output stage amplifies half cycles of an analog AC signal, and the other section amplifies the other half cycles of the AC signal.
In a conventional power amplifier, the two sections of an SEPP stage comprise two power transistors with at least one transistor being wired in an emitter-follower circuit configuration, and with a biasing circuit positioned as a pre-stage of the SEPP section. The conventional power amplifier, however, has several disadvantages mentioned below.
First, the biasing circuit is complicated, especially for an SEPP stage constructed by an emitter-grounded configuration at both sections.
Second, the power transistors are apt to be harmed by over current flowing when a large input signal is added so that the amplifier needs some form of protection circuit.
Third, the power transistors enter a cutoff condition when there is no signal at the base terminals of the transistors, so that the output signal includes a clipping noise.
Fourth, output amplitude is limited due to the use of an emitter-follower configuration, with a resistor for the biasing circuit or a resistor for an idling current circuit.
Accordingly, an object of the present invention is to provide a power amplifier with an improved biasing circuit.
A further object of the present invention is to provide a power amplifier which prevents an over current flowing through the power transistors.
Another object of the present invention is to provide a power amplifier with a small current flowing through the power transistors when there is no signal at the base of the power transistors.
A still further object of the present invention is to provide a power amplifier having a large output efficiency.